You want to know why the girls don't mention Spike at all in this comic? I can give you two possible interpretations why:
1. How many teenage/young adult people do you know who would actually care about their best friend's annoying douchebag kid brother? As far as I know in both TV and real life, the answer is a big fat zero.
and 2. I know this contradicts my first reply to you, but did it not occur to you that Twilight might have left Spike in charge of the library while she was getting her Princess training because she knew that he couldn't bear to be separated from the friends he's made since they've arrived and they're on their way to see him? You immediately assume the worst about them without considering the other alternatives as to why he's not mentioned.
The sad thing is, you and other fans who complain about Spike being missing from fan content are actually starting to make me hate him more as a character when before, his actions in "Bridle Gossip" were the only thing I held against him. It gets especially worse when you consider that you're taking a minor piece of a non-serious joke comic seriously, which give me the impression that you're nothing but a humorless troll.